This invention relates to a servo control circuit, and more particularly to a head drum servo circuit for a video tape recorder.
Video tape recorders (called hereafter VTRs) include one or more rotary magnetic heads for recording a television signal on a magnetic tape. Although numerous types of VTRs are known differing in the number of rotary magnetic heads and in the manner of wrapping the video tape on the guide drum, they all include a drum servo circuit for controlling the position of their rotary magnetic head or heads and a capstan servo circuit for controlling transport of the video tape past the head drum.
The drum servo circuit establishes and maintains a predetermined rotational phase relationship between the rotary magnetic heads and a reference signal.
One problem with conventional drum servo circuits is that their servo control characteristics change with aging and/or temperature variations. After a head drum motor is used for a long time, for example, more than 1000 hours, its response to a particular error voltage from the drum servo circuit is different from its response when it was new.
An additional problem arises from the fact that there are two principal tape formats called "format A" and "format B" which differ from each other in the location of the point on the tape at which RF switching between magnetic heads takes places. As is well known, such locations of RF switching correspond to reference timing signals for the drum servo circuit. Accordingly, the positions of vertical synchronizing signals recorded on the video tape are slightly different in formats A and B. When a video signal having format A or B is edited onto a video tape which contains a video signal previously recorded by a VTR using the other format, the positions of the vertical synchronizing signals are discontinuous or disordered at the editing cut-in point. When the edited video tape is reproduced, the reproduced image fluctuates at the cut-in point.